For many years decorative high pressure laminates have been used as a surfacing material in residential and commercial structures where aesthetic effects are desired in combination with functional behavior, such as wear, heat and stain resistance. Typical applications are for walls, partitions, table tops, counter tops, furniture, doors and many other uses. These decorative laminates have been produced by a plurality of prior art processes. In making such laminates it is conventional to utilize a plurality of resin impregnated core sheets generally composed of kraft paper which have been impregnated with a thermosetting resin and more particularly as a general rule with a thermosetting phenolic resin. When the kraft has been impregnated with a thermosetting resin, the sheets are dried and cut to appropriate size. Thereupon, a plurality of these resin impregnated sheets are stacked in a superimposed relationship. The number of plies in the stack will depend upon the ultimate intended use of the laminate. For most purposes the number of these core sheets will total about 6 to 9. For decorative laminates there is then placed on the stack of core sheets a decorative sheet which is generally a sheet of alphacellulose paper bearing a printed design or a light color and impregnated with a noble thermosetting resin (see U.S. Pat. No. 3,373,068 and 3,418,189) which is not subject to any significant darkening upon the application of heat. Resins for the decorative sheets are the aminotriazine resins and additionally the unsaturated polyester resins, the epoxy resins and the like.
It is generally desirable when making decorative laminates to make use of a protective overlay sheet which is similar to the decorative sheet but generally devoid of design and in the final laminate is transparent. The laminates produced are heat and pressure consolidated to a unitary structure in which the press plate is a polished stainless steel plate. Such an approach produces a very smooth surface laminate with a glossy finish. Other techniques are used to produce laminates with a smooth surface but with a satin or brush textured effect. More recent techniques permit the manufacture of laminates with a three dimensional design on the surface.
These decorative laminates are usually bonded with an adhesive to a smooth surface rigid substrate such as plywood, particleboard, gypsum board, metal, cement and the like. Typical uses for the above is for applications such as partitions, walls, panels, doors, cabinets, table tops, counter tops, desks, furniture and many other uses.
Over the years the adhesives that have normally been used in bonding the aforementioned decorative laminate to substrates consist essentially of rigid, semi-rigid and the elastomeric types that have bonding properties which are compatible with the heat and pressure limitations of the laminate and capable of resisting humidity and temperature variations and induced stresses inherent in a bonded laminate to substrate assemblies, particularly in applications such as bathrooms and kitchens.
The rigid setting adhesives are those based usually on either urea-formaldehyde resins or resorcinol-formaldehyde resins and they produce excellent bonds and adequate water resistance. The adhesive is applied either to the back side of the laminate or the coating is applied to the substrate. These adhesives are usually hot set but can be used at room temperature.
The assembly is then either clamped or inserted in a veneer press and low pressure of approximately 25 psi is applied. These adhesives are considered "hot set" and will cure to excellent bonds by the application of heat of about 200.degree. F. for 7 to 10 minutes. In the absence of heat, for example at room temperature conditions, satisfactory workable bonds are achieved under pressure in about 7 to 12 hours. "Workable" bond means that the panel may be trimmed, sawed and routed.
The semi-rigid adhesives encompass such materials as polyvinyl acetate emulsions, polyvinyl alcohol, epoxies and others.
Like the rigid type adhesives discussed above, the adhesive is either coated on the back of the laminate or on the substrate to be covered. Excellent bonds are achieved after subjecting the veneered assembly to clamp pressures or veneering equipment pressure for a period of 30 minutes up to about 8 hours at room temperature.
With the introduction of elastomeric adhesives some years ago and more specifically the "contact" type, which are solutions of rubbery polymers, it became possible to produce workable bonds almost instantly upon contact. Their ability to be processed rapidly and produce bonds without mechanical equipment justifies their use. Usually, pressure from a hand roller is all that is required to achieve excellent bonds. The use of "contact" type adhesive has significantly enlarged the scope of applications of high pressure decorative laminates such as had not been possible heretofore, namely, to on-site applications.
More recently products have appeared in the marketplace for on-site applications making use of a decorative laminate bonded to a plastic foam core. This type of product may be bonded with an elastomeric adhesive to a relatively non-smooth wall, such as ceramic tile, rough surface concrete and gypsum board. A panel of this type is light in weight, has good moisture resistance and because of the inherent nature of the foam, will bridge over substrate surface irregularities to the decorative surface of the laminate. However, panels and installations of the foregoing kind have certain shortcomings and leave much to be desired because of erratic or failure in bonding of the laminate to the plastic foam, as well as the plastic foam to the substrate, inadequate bridging of the substrate's surface irregularities leaving an unsightly, unpleasant and unattractive installation and stress cracking of the decorative surface from hardware cut-outs making a replacement necessary. These shortcomings have now been overcome.